New in Symfony 4.3: Automatic Search Engine Protection

Search engines like Google, DuckDuckGo, Baidu and Yandex do a great job crawling
and indexing your web sites and applications. However, this is not so great when
you accidentally publish a development version of your app.

In Symfony 4.3 we improved this situation by disallowing the search engine
indexing for development applications. How does it work? If the app kernel
runs in debug mode (by default this happens when the Symfony environment is
not prod) Symfony adds a X-Robots-Tag: noindex HTTP header to all the
responses.

The X-Robots-Tag header is one of the HTTP headers that legit search engines
take into account when crawling a web site. If your own app already added that
header, this new feature doesn't change its value. Also, if you don't like this
feature, you can disable it with this config option: